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    <h1>
      Handling Events
    </h1>
    <div class="subHeader"></div>

    <p>Handling events with React elements is very similar to handling events on DOM elements. There are some syntactic differences:</p>

<ul>
<li>React events are named using camelCase, rather than lowercase.</li>
<li>With JSX you pass a function as the event handler, rather than a string.</li>
</ul>

<p>For example, the HTML:</p>
<div class="highlight"><pre><code class="language-html" data-lang="html"><span class="nt">&lt;button</span> <span class="na">onclick=</span><span class="s">&quot;activateLasers()&quot;</span><span class="nt">&gt;</span>
  Activate Lasers
<span class="nt">&lt;/button&gt;</span>
</code></pre></div>
<p>is slightly different in React:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="hll"><span class="o">&lt;</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="nx">activateLasers</span><span class="p">}</span><span class="o">&gt;</span>
</span>  <span class="nx">Activate</span> <span class="nx">Lasers</span>
<span class="o">&lt;</span><span class="err">/button&gt;</span>
</code></pre></div>
<p>Another difference is that you cannot return <code>false</code> to prevent default behavior in React. You must call <code>preventDefault</code> explicitly. For example, with plain HTML, to prevent the default link behavior of opening a new page, you can write:</p>
<div class="highlight"><pre><code class="language-html" data-lang="html"><span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">&quot;#&quot;</span> <span class="na">onclick=</span><span class="s">&quot;console.log(&#39;The link was clicked.&#39;); return false&quot;</span><span class="nt">&gt;</span>
  Click me
<span class="nt">&lt;/a&gt;</span>
</code></pre></div>
<p>In React, this could instead be:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">ActionLink</span><span class="p">()</span> <span class="p">{</span>
<span class="hll">  <span class="kd">function</span> <span class="nx">handleClick</span><span class="p">(</span><span class="nx">e</span><span class="p">)</span> <span class="p">{</span>
</span><span class="hll">    <span class="nx">e</span><span class="p">.</span><span class="nx">preventDefault</span><span class="p">();</span>
</span><span class="hll">    <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">&#39;The link was clicked.&#39;</span><span class="p">);</span>
</span><span class="hll">  <span class="p">}</span>
</span>
  <span class="k">return</span> <span class="p">(</span>
<span class="hll">    <span class="o">&lt;</span><span class="nx">a</span> <span class="nx">href</span><span class="o">=</span><span class="s2">&quot;#&quot;</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="nx">handleClick</span><span class="p">}</span><span class="o">&gt;</span>
</span>      <span class="nx">Click</span> <span class="nx">me</span>
    <span class="o">&lt;</span><span class="err">/a&gt;</span>
  <span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
<p>Here, <code>e</code> is a synthetic event. React defines these synthetic events according to the <a href="https://www.w3.org/TR/DOM-Level-3-Events/">W3C spec</a>, so you don&#39;t need to worry about cross-browser compatibility. See the <a href="/react/docs/events.html"><code>SyntheticEvent</code></a> reference guide to learn more.</p>

<p>When using React you should generally not need to call <code>addEventListener</code> to add listeners to a DOM element after it is created. Instead, just provide a listener when the element is initially rendered.</p>

<p>When you define a component using an <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes">ES6 class</a>, a common pattern is for an event handler to be a method on the class. For example, this <code>Toggle</code> component renders a button that lets the user toggle between &quot;ON&quot; and &quot;OFF&quot; states:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">Toggle</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
  <span class="nx">constructor</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
    <span class="kr">super</span><span class="p">(</span><span class="nx">props</span><span class="p">);</span>
    <span class="k">this</span><span class="p">.</span><span class="nx">state</span> <span class="o">=</span> <span class="p">{</span><span class="nx">isToggleOn</span><span class="o">:</span> <span class="kc">true</span><span class="p">};</span>

<span class="hll">    <span class="c1">// This binding is necessary to make `this` work in the callback</span>
</span><span class="hll">    <span class="k">this</span><span class="p">.</span><span class="nx">handleClick</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">handleClick</span><span class="p">.</span><span class="nx">bind</span><span class="p">(</span><span class="k">this</span><span class="p">);</span>
</span>  <span class="p">}</span>

<span class="hll">  <span class="nx">handleClick</span><span class="p">()</span> <span class="p">{</span>
</span><span class="hll">    <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">(</span><span class="nx">prevState</span> <span class="o">=&gt;</span> <span class="p">({</span>
</span><span class="hll">      <span class="nx">isToggleOn</span><span class="o">:</span> <span class="o">!</span><span class="nx">prevState</span><span class="p">.</span><span class="nx">isToggleOn</span>
</span><span class="hll">    <span class="p">}));</span>
</span><span class="hll">  <span class="p">}</span>
</span>
  <span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="p">(</span>
<span class="hll">      <span class="o">&lt;</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">handleClick</span><span class="p">}</span><span class="o">&gt;</span>
</span>        <span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">isToggleOn</span> <span class="o">?</span> <span class="s1">&#39;ON&#39;</span> <span class="o">:</span> <span class="s1">&#39;OFF&#39;</span><span class="p">}</span>
      <span class="o">&lt;</span><span class="err">/button&gt;</span>
    <span class="p">);</span>
  <span class="p">}</span>
<span class="p">}</span>

<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
  <span class="o">&lt;</span><span class="nx">Toggle</span> <span class="o">/&gt;</span><span class="p">,</span>
  <span class="nb">document</span><span class="p">.</span><span class="nx">getElementById</span><span class="p">(</span><span class="s1">&#39;root&#39;</span><span class="p">)</span>
<span class="p">);</span>
</code></pre></div>
<p><a href="http://codepen.io/gaearon/pen/xEmzGg?editors=0010">Try it on CodePen.</a></p>

<p>You have to be careful about the meaning of <code>this</code> in JSX callbacks. In JavaScript, class methods are not <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_objects/Function/bind">bound</a> by default. If you forget to bind <code>this.handleClick</code> and pass it to <code>onClick</code>, <code>this</code> will be <code>undefined</code> when the function is actually called.</p>

<p>This is not React-specific behavior; it is a part of <a href="https://www.smashingmagazine.com/2014/01/understanding-javascript-function-prototype-bind/">how functions work in JavaScript</a>. Generally, if you refer to a method without <code>()</code> after it, such as <code>onClick={this.handleClick}</code>, you should bind that method.</p>

<p>If calling <code>bind</code> annoys you, there are two ways you can get around this. If you are using the experimental <a href="https://babeljs.io/docs/plugins/transform-class-properties/">property initializer syntax</a>, you can use property initializers to correctly bind callbacks:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">LoggingButton</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
<span class="hll">  <span class="c1">// This syntax ensures `this` is bound within handleClick.</span>
</span><span class="hll">  <span class="c1">// Warning: this is *experimental* syntax.</span>
</span><span class="hll">  <span class="nx">handleClick</span> <span class="o">=</span> <span class="p">()</span> <span class="o">=&gt;</span> <span class="p">{</span>
</span><span class="hll">    <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">&#39;this is:&#39;</span><span class="p">,</span> <span class="k">this</span><span class="p">);</span>
</span><span class="hll">  <span class="p">}</span>
</span>
  <span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="p">(</span>
      <span class="o">&lt;</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">handleClick</span><span class="p">}</span><span class="o">&gt;</span>
        <span class="nx">Click</span> <span class="nx">me</span>
      <span class="o">&lt;</span><span class="err">/button&gt;</span>
    <span class="p">);</span>
  <span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
<p>This syntax is enabled by default in <a href="https://github.com/facebookincubator/create-react-app">Create React App</a>.</p>

<p>If you aren&#39;t using property initializer syntax, you can use an <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions">arrow function</a> in the callback:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">LoggingButton</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
  <span class="nx">handleClick</span><span class="p">()</span> <span class="p">{</span>
    <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">&#39;this is:&#39;</span><span class="p">,</span> <span class="k">this</span><span class="p">);</span>
  <span class="p">}</span>

  <span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
<span class="hll">    <span class="c1">// This syntax ensures `this` is bound within handleClick</span>
</span><span class="hll">    <span class="k">return</span> <span class="p">(</span>
</span><span class="hll">      <span class="o">&lt;</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{(</span><span class="nx">e</span><span class="p">)</span> <span class="o">=&gt;</span> <span class="k">this</span><span class="p">.</span><span class="nx">handleClick</span><span class="p">(</span><span class="nx">e</span><span class="p">)}</span><span class="o">&gt;</span>
</span>        <span class="nx">Click</span> <span class="nx">me</span>
      <span class="o">&lt;</span><span class="err">/button&gt;</span>
    <span class="p">);</span>
  <span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
<p>The problem with this syntax is that a different callback is created each time the <code>LoggingButton</code> renders. In most cases, this is fine. However, if this callback is passed as a prop to lower components, those components might do an extra re-rendering. We generally recommend binding in the constructor or using the property initializer syntax, to avoid this sort of performance problem.</p>


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